1. Field of the Invention
This invention relates to propane oxidation. In one aspect the invention relates to an improved reactor for propane oxidation while in another aspect, the invention relates to an improved process for propane oxidation.
2. Description of the Related Art
In the direct oxidation of propane to acrylic acid (AA) process, propionic acid (PA) is formed as an undesirable side product. In one embodiment of the propane to AA process, the preferred catalyst system consists of mixed metal oxides of molybdenum, vanadium, tellurium and niobium (Mo/V/Te/Nb). These catalysts produce PA at levels that may range from over 1,000 parts per million (ppm) to less than 10,000 ppm when operating under conditions to achieve maximum AA yield (equal to or greater than (≧) 85% oxygen conversion).
In addition, excess propionic acid byproduct can, upon esterification of the AA product, impart undesirable characteristics such as high volatile organic content (VOC), odor or color to the acrylate ester (AE) and its corresponding polymer products. Typically, PA specifications for AA product streams from conventional propylene oxidation, prior to esterification, range from 500-1000 ppm, well below the levels seen in the propane oxidation product. Thus, in order for propane oxidation to be more economically desirable, PA byproduct levels need to be reduced either through the oxidation step or in downstream separation steps.
The separation of propionic acid from acrylic acid is problematic. The boiling points of both are less than (<) 1° C. apart, and they are not capable of separation by distillation. Although methods exist for PA separation, e.g., melt crystallization, these steps substantially increase the capital and operating cost of the AA purification process by requiring additional equipment and utilities to effect the desired separation. Thus it would be advantageous and desirable to be able to control the AA process in such a way as to reduce the formation of PA such that additional costly purification is substantially reduced.